1. Field of the Invention
The present invention relates generally to a computer-controlled system and method for sorting items. More specifically, the present invention relates to a computer-controlled system for sorting recyclable waste items such as plastics and glass.
2. Related Art
Solid waste material of the type exemplified by municipal waste and more specifically plastic and/or glass waste have traditionally presented problems of disposal. The disposal problem has become increasingly critical in recent years as a result of increased production of plastic waste items.
Conventionally, such plastic waste items have been disposed of by such means as incineration and landfill. Incinerating and landfill operations present significant environmental problems not to mention depletion of valuable resources. As a result, there exists a great need to recycle plastic waste items.
Plastic waste items typically take the form of plastic bottles. However, many other items too numerous to specify here are made from plastic materials.
To make plastic recycling effective, it is necessary to segregate the returned plastic items into groups whose scrap price is inherently higher then unsegregated plastic items. In other words, it is necessary to separate the polyethylene terephthalate (PET) items from the polyvinyl chloride (PVC) items or from the high density polyethylene (HDPE) items, etc. In addition to separating by type of plastic, it is often necessary to further separate a specific type of plastic item by its color. Plastic items further segregated by their color are even more valuable. This is because recycling a PVC item with a PET item will not result in a pure PVC or PET end product. As a result, the recycled polymer (a combination of PVC and PET) cannot be widely used by the plastic industry. Therefore, for recycling to be most cost effective, it is necessary for example to separate PVC from PET, and further, green PET from clear PET: type and color.
Several practical issues need to be addressed when sorting plastic waste items. One such concern relates to the size of the plastic item to be examined. Plastic waste items often are initially packed together in a crushed form in what is termed a "bail." A typical bail is about four feet by four feet by six feet (4.times.4.times.6 ft.). These bails are broken apart by what is known in the art as a "bail breaker."
As a result of this bail breaking operation, the plastic items although broken apart may be resting one on top of the other. Moreover, the plastic waste items may have been cut into smaller pieces which range in size and configuration. Any sortation system must be able to handle a variety of sized plastic items and to disregard items which are too small or large. In addition, as a result of the bail breaking operation, any sortation system must also be able to fully separate the plastic waste items determined examinable.
An additional concern is that plastic waste items are typically a composite of several types of plastics. By way of example only, a typical soda (soft drink) container may have a bottom cap which is made of a plastic material different than the container material and/or the cap material. In these circumstances, it is often desirable to sort the plastic waste items based on the dominant type of plastic present.
Conventional sorting of plastic waste items has often been performed using manual means. As will be readily evident, a manual based sorting system is very labor intensive, making recycling costs high. Recycling costs are high because the speed at which the plastic items can be sorted is slow. Additionally, it is difficult to find people to do such work and turnover of such people is high. Moreover, manual sorting is susceptible to error. A person can easily miss-classify a given piece of plastic. Improper sorting significantly reduces the value of the recovered plastics due to the impurities caused by the missorted items as discussed above.
Sorting systems have also been developed that sort plastic by x-ray detection. An x-ray based sorting system has obvious health related drawbacks. Another problem inherent with x-ray detection or similar systems is that of singulation and orientation. In order to properly examine a plastic item, the plastic items must be spaced a sufficient distance apart so that proper examination can be made. Furthermore, the plastic items should be properly situated so that accurate examination can be made.
Sorting systems have also been developed that sort plastic items using hydrocyclones and flotation tanks employing water and alcohol based media. Systems of this type separate dissimilar plastics by specific gravity using centrifugal forces. Systems of this type also have significant drawbacks. One such disadvantage is that specific gravity based systems are not very accurate. The degree of sortation in such a system is small allowing only a relatively few types of plastic items to be separated. Moreover, such systems sort the plastic items slowly.